Visual experience is one of the factors which influences the formation of nerve connections in the developing brain. A dramatic example of this effect occurs in the development of binocular input to the tectum of the frog, Xenopus laevis. In this species, the abnormal experience which results from abnormal positioning of the eye in its socket can lead to a major alteration in the location of terminals from the nucleus isthmi (NI), a midbrain structure which projects to the tectum. However, there is an apparent discrepancy between the results of studies using electrophysiological methods and those using the horseradish peroxidase anatomical tracing technique. Two possible explanations for this difference will be tested. One of the explanations is that the electrophysiological results are completely explicable as the result of abnormal anatomical connections; the alternative is that parts of the system are physiologically silent ("suppressed") while other parts are active. Anterograde labelling of isthmo-tectal arbors at various stages of development will be used to test the hypothesis that the final pattern of isthmo-tectal topography is brought about by selective retention of physiologically appropriate parts of an initially widespread axonal arbor. Studies of the time of cell genesis in the NI will be used to test the alternative hypothesis that selective cell death is a major mechanism in determining the topography of isthmo-tectal connections. A third topographic input to the tectum, the uncrossed isthmo-tectal projection, will be investigated electrophysiologically, by lesion studies, and electron microscopically, in order to determine whether it is necessary for the formation of an appropriately oriented map from the crossed isthmo-tectal projection. It has been proposed that noradrenergic innervation is necessary for plasticity in the visual cortex (Kasamatsu and Pettigrew, 1979). Comparable studies of the effect of destruction of the NE input to the tectum are underway and will be continued. The role of the NI in visuomotor behavior will also be studied.